Take a deep breath and slowly exhale. The oxygen in that breath is being used by your mitochondria in reactions that produce ATP from sugars and other food molecules you ate. Where did that oxygen come from originally?

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Plants produce the oxygen via the process of photorespiration, in which they break down sugars at night when photosynthesis cannot occur.
Plants produce the oxygen via the process of photorespiration when they break down sugars in their mitochondria.
The oxygen in the atmosphere is produced by the breakdown of carbon dioxide during photosynthesis.
The oxygen in the air is produced by the splitting of water during the light-dependent reactions of photosynthesis.